1. Field of the Invention
The present invention relates generally to the field of electrosurgery and, more particularly, to surgical devices and methods which employ very high frequency electrodes comprising an array of individual, isolated electrode terminals.
The field of electrosurgery includes a number of loosely related surgical techniques which have in common the application of electrical energy to modify the structure or integrity of patient tissue. Electrosurgical procedures usually operate through the application of very high frequency currents to cut or ablate tissue structures, where the operation can be monopolar or bipolar. Monopolar techniques rely on external grounding of the patient, where the surgical device defines only a single electrode pole. Bipolar devices comprise both electrodes for the application of current between their surfaces.
Electrosurgical procedures and techniques are particularly advantageous since they generally reduce patient bleeding and trauma associated with cutting operations. Additionally, electrosurgical ablation procedures, where tissue surfaces and volume may be reshaped, cannot be duplicated through other treatment modalities.
The use of electrosurgical procedures in electrically conductive environments, however, can be problematic. For example, many arthroscopic procedures require flushing of the region to be treated with isotonic saline (also referred to as normal saline), both to maintain an isotonic environment and to keep the field of viewing clear. The presence of saline, which is a highly conductive electrolyte, can cause shorting of the electrosurgical electrode in both monopolar and bipolar modes. Such shorting causes unnecessary heating in the treatment environment and can further cause non-specific tissue destruction.
Present electrosurgical techniques used for tissue ablation also suffer from an inability to control the depth of necrosis in the tissue being treated. Most electrosurgical devices rely on creation of an electric arc between the treating electrode and the tissue being cut or ablated to cause the desired localized heating. Such arcs, however, often create very high temperatures causing a depth of necrosis greater than 500 xcexcm, frequently greater than 800 xcexcm, and sometimes as great as 1700 xcexcm. The inability to control such depth of necrosis is a significant disadvantage in using electrosurgical techniques for tissue ablation, particularly in arthroscopic procedures for ablating and/or reshaping fibrocartilage, articular cartilage, meniscal tissue, and the like.
In an effort to overcome at least some of these limitations of electrosurgery, laser apparatus have been developed for use in arthroscopic and other procedures. Lasers do not suffer from electrical shorting in conductive environments, and certain types of lasers allow for very controlled cutting with limited depth of necrosis. Despite these advantages, laser devices suffer from their own set of deficiencies. In the first place, laser equipment can be very expensive because of the costs associated with the laser light sources. Moreover, those lasers which permit acceptable depths of necrosis (such as excimer lasers, erbium:YAG lasers, and the like) provide a very low volumetric ablation rate, which is a particular disadvantage in cutting and ablation of fibrocartilage, articular cartilage, and meniscal tissue. The holmium:YAG and Nd:YAG lasers provide much higher volumetric ablation rates, but are much less able to control depth of necrosis than are the slower laser devices. The CO2 lasers provide high rate of ablation and low depth of tissue necrosis, but cannot operate in a liquid-filled cavity.
For these reasons, it would be desirable to provide improved apparatus and methods for efficiently cutting and ablating tissue, particularly fibrocartilage, articular cartilage, meniscal tissue, and the like in arthroscopic and other procedures. Such apparatus and methods should be able to selectively cut and ablate tissue and other body structures in electrically conductive environments, particularly regions which are filled with blood, irrigated with saline, or the like. Such apparatus and methods should be able to perform cutting and ablation of tissues, particularly fibrocartilage, articular cartilage, meniscal tissue, and the like, while limiting the depth so necrosis and tissue adjacent to the treatment site. Such apparatus and methods should be amenable to precise control over the energy flux levels applied to the treatment region, and should be able to provide energy densities sufficient to provide rapid cutting and ablation. The devices should be adaptable to a wide variety of purposes, particularly including both small and large electrode surfaces, and rigid and flexible structures which can be used in open surgery, arthroscopic surgery, and other minimally invasive surgical techniques.
2. Description of the Background Art
Devices incorporating radio frequency electrodes for use in electrosurgical and electrocautery techniques are described in Rand et al. (1985) J. Arthro. Surg. 1:242-246 and U.S. Pat. Nos. 5,281,216; 4,943,290; 4,936,301; 4,593,691; 4,228,800; and 4,202,337. U.S. Pat. No. 5,281,216 describes a bipolar device having an active electrode coated with a high impedance material where the differential impedance between the active and return electrodes is optimized to provide a desired cutting effect. Vascular catheters and devices incorporating radio frequency electrodes to assist in penetrating atheroma and plaque are described in U.S. Pat. Nos. 5,281,218; 5,125,928; 5,078,717;4,998,933; and 4,976,711, and PCT publications WO 93/20747 and WO 90/07303, the latter of which describes a catheter having four isolated electrode surfaces at its distal end. Electrosurgical power supplies including power controls and/or current limiting systems are described in U.S. Pat. No. 5,267,997 and PCT publication WO 93/20747. Surgical lasers for cutting and ablation in arthroscopic and other procedures are described in Euchelt et al. (1991) Surgery and Medicine II: 271-279; and U.S. Pat. Nos. 5,147,354; 5,151,098; 5,037,421; 4,968,314; 4,785,806; 4,737,678; 4,736,743; and 4,240,441.
The present invention provides methods and apparatus for selectively applying electrical energy to structures within a patient""s body. The methods and apparatus are particularly useful for performing electrosurgical interventions, such as ablation and cutting of body structures, through the controlled application of high frequency electrical voltages and currents.
Apparatus according to the present invention comprise electrosurgical probes including a shaft having a proximal end, a distal end, an electrode array disposed near the distal end of the shaft, and a connector disposed near the proximal end of the shaft. The shaft will be of a type suitable for use in open and/or minimally invasive surgical procedures, such as arthroscopic, laparoscopic, thoracoscopic, and other endoscopic procedures. The shaft may be rigid, flexible, or include both rigid and flexible portions, and will be generally suitable for manipulation by the treating physician from the proximal end. A common electrode may optionally be provided on the shaft, typically being mounted over the exterior of the shaft and spaced proximally from the electrode array, and preferably being covered with a perforate, electrically non-conductive shield to protect against accidental tissue contact. The electrode array includes a plurality of electrically isolated electrode terminals disposed over a contact surface, which may be a planar or non-planar surface and which may be located at the distal tip or over a lateral surface of the shaft, or over both the tip and lateral surface(s). Such electrode arrays are particularly useful for performing electrosurgical ablation, as described in more detail below. In addition to planar and other surfaces, the electrode array may be arranged in a linear pattern, which is particularly useful as a blade for electrosurgical cutting procedures. The electrode array will include at least two and preferably more electrode terminals, and may further comprise a temperature sensor. The connector permits electrical coupling of the electrode terminals, and optionally temperature sensor, to a high frequency power supply and optionally temperature monitor and/or controller or operation of the probe.
The use of such electrode arrays in electrosurgical procedures is particularly advantageous as it has been found to limit the depth of tissue necrosis without substantially reducing power delivery and ablation rates. Heretofore, increased power delivery with electrosurgical devices has generally been achieved by increasing monolithic electrode area. The resulting large electrode surfaces, however, cause tissue necrosis to a depth which varies proportionally with the width and area of the electrode surface. The present invention provides a more controlled necrosis depth by utilizing a plurality of isolated electrode terminals, where the terminals are preferably laterally spaced-apart by a distance from one-tenth to one terminal diameter, with spacing between larger electrode terminals generally being at the lower end of the range. Such spacing provides adequate power delivery and ablation rates without excessive tissue necrosis, which is usually limited to a depth less than one electrode terminal diameter.
Apparatus according to the present invention further include an electrosurgical high frequency power supply comprising a multiplicity of independent current sources and a connector which mates with a corresponding connector on the electrosurgical probe. The current sources preferably comprise passive or active current limiting circuit structures in parallel with each other and in series with a common voltage source within the power supply. Passive current limiting circuit structures may include inductor(s), capacitor(s), and/or resistor(s) in known circuit configurations. In all cases, the passive current limiting structures will be designed to limit current flow when the associated electrode terminal is in contact with a low resistance return path back to the common or return electrode. Preferred passive current limiting structures comprise (1) inductors in series with each electrode terminal and (2) capacitors in series and inductors in parallel with each electrode terminal, as described in detail hereinafter.
Active current limiting circuit structures will usually comprise a switching element to turn off current flow whenever the associated electrode terminal contacts a low (or in some instances high) impedance return path back to the common or return electrode. The switching element could be mechanical, e.g., a relay, but preferably will be solid state, e.g., a silicon controlled rectifier (SCR) or silicon controlled switch (SCS). The switch will be turned on and off by a controller which can detect the low resistance path (typically by sensing current flow above a threshold value). The controller can be implemented in hardware or software, typically being part of the power supply.
The high frequency electrosurgical power supply optionally includes a temperature controller which is connected to the temperature sensor on the electrosurgical probe and which adjusts the output voltage of the voltage source in response to a temperature set point and the measured temperature value received from the probe. In this way, the power output and temperature may be controlled while the individual current sources limit or block the power output from corresponding individual electrode terminals. Such limitation of individual electrode terminal power outputs is critical to limiting energy loss from the electrode array as described in more detail below.
The present invention still further provides an electrosurgical system including both the electrosurgical probe and electrosurgical power supply as described above.
According to the method of the present invention, an electrosurgical probe is positioned adjacent to a body structure so that an electrode array is brought into at least partial contact with the structure. The electrode array includes a plurality of isolated electrodes, and a high frequency voltage is applied between the electrode array and the patient""s body. The voltage causes current flow between each electrode terminal and the body structure which is through all low electrical impedance paths is preferably but not necessarily limited. It will be appreciated that such low impedance paths generally occur when an electrode terminal does not contact the body structure, but rather is in contact with a low impedance environment, such as saline, blood, or other electrolyte. The presence of an electrolyte provides a relatively low impedance path back to the common or return electrode, which may be on the electrosurgical probe or may be attached externally to the patient. Such electrosurgical methods are particularly useful when a region is to be flushed with saline, such as in an electrosurgical ablation of fibrocartilage, articular cartilage, meniscal tissue, and the like, in arthroscopic procedures.
In some cases, it may be desirable to provide current limitation or control when individual electrode terminals contact very high resistance body structures, such as bone, cartilage (which has a higher resistivity than meniscus and other tissues), and the like. Current limitation when the electrode terminals contact high resistance structures will usually require active control schemes (i.e., passive control circuitry will be inadequate), and it will be possible to provide control protocols where current can be limited when it either exceeds or falls below an expected range characteristic of the target tissue to be treated.